436 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



The wland lies between the epithelium and connective tissue of 

 the lining membx-ane of the gill-chamber in its posterior dorsal por- 

 tion. The bulk of the organ lies above the dorso-median angle of 

 the chamber extending upwards as a lobe between the trapezius and 

 lev. branchicdes muscles, and attaining a thickness of 700 //, or 

 eighteen times that of the epithelium, and one-tenth of the vertical 

 median diameter of this part of the head. From this thickening 

 the gland thins out laterally and medially terminating on a line with 

 the floor of the brain-case. Its anterior margin is on a line with 

 the third branchial arch, and it terminates behind, slightly in front 

 of the transverse process of the supraclavicle. The cavity in the 

 glard, shown in the figures, is a mere split in the tissue and without 

 a limiting membrane. 



The substance of the gland consist of connective tissue fibres 

 mostly parallel to the epithelium and small round nucleated cells not 

 larger than 4 p.. They are readily distinguished from the epithelial 

 cells with which they are in contact by their smaller size and the 

 deeper stain imparted to them by various reagents. There are no 

 blood spaces and the tissue is homogeneous throughout, except that 

 it is looser toward the centre of the gland where the split occurs. 



The gland was secured in the adult by removing the entire mem- 

 brane and examined by cutting sections. The greatest thickness 

 observed in four specimens was exactly that given above for the 

 young fish, and it may be safely stated that in the full grown fish it 

 is absolutely smaller. The connective tissue covering it above con- 

 tains fat cells, and at places exceeds the gland in thickness. It 

 sends processes through to the epithelium at' right angles to its sur- 

 face. This reticulate connective tissue appears to gradually increase 

 while the cellular elements decrease, and in places undergo fatty 

 degeneration. 



The thymus gland in Amiurus is, therefore, an embryonic struc- 

 ture, while the thyroid developes and is functional in the adult 

 animal. The former is, no doubt, developed as a diverticulum from 

 the epithelium of the branchial cavity as the latter is from the 

 mouth. 



It is interesting to find a member of such an old family as the 

 Siluroids possessing all those structures (pseudobranchia thyroid 

 thymus and head-kidney) which are not, according to our present 

 knowledge, constant in their occurrence in fishes, and have been 



